1. Technical Field
The disclosure relates generally to wireless communication.
2. Description of Related Art
In wireless communication, a communication circuit, such as a receiver or a transmitter, is employed for modulating information to be transmitted, and transmitting the modulated information via an antenna or demodulating a signal received via the antenna, and extracting the information carried by the demodulated signal. A conventional transmitter can be implemented with various architectures, one of which is direct up-conversion transmitter. A simplified block diagram of the direct-up transmitter is illustrated in FIG. 1. As illustrated in FIG. 1, the direct-up transmitter 100 comprises an in-phase branch (I-branch) 110 and a quadrature-phase branch (Q-branch) 120, which respectively comprise digital-to-analog converters (DAC) 111 and 121, low-pass filters (LPF) 112 and 122, mixers 113 and 123, an adder 130, a power amplifier (PA) 140 and an antenna 150. In the I-branch 110, a digital baseband in-phase signal BBIt will be inputted to the DAC 111 for conversion, and then be inputted to the LPF 112 for filtering. Subsequently, it is mixed with a local in-phase oscillation signal LOIt by the mixer 113, so as to generate an in-phase analog radio frequency AnIt. In the Q-branch 120, a digital baseband quadrature-phase signal BBIt is also processed in a similar manner, and is subsequently mixed with a quadrature-phase local oscillating signal LOQt by the mixer 123 to generate a quadrature-phase analog radio frequency AnQt. Accordingly, with the adder 130, in-phase analog radio frequency AnIt and the quadrature-phase analog radio frequency AnQt will be summed up. The summed signal will be amplified by the power amplifier 140 and transmitted out through antenna 150.
The direct-conversion transmitter has inherent advantages in low cost, small package size and low power consumption such that it is widely used in a variety of wireless communication devices. The tradeoff is a higher degree of radio frequency imperfection, especially, in-phase (I) and quadrature (Q) mismatch in the analog front-end, which means the amplitude, phase, or path (i.e., delay time) mismatches between the I-branch signal and Q-branch signal. For example, in single-carrier modulation system, the amplitude mismatch between I-branch and Q-branch signal results in a visible distortion in the constellation—the square constellation of a 64-QAM signal would become rectangular. Besides, the I/Q mismatch introduces unwanted image interference and severely limits the achievable SNR in the system, which results in loss of information and degrades error vector magnitude (EVM) or bit error rate (BER).
U.S. application Publication NO. 20020015450 discloses a method and an arrangement for determining correction parameters used to calibrate the phase and amplitude mismatch of an I/Q modulator in a transmitter. The transmitter includes an I/Q modulator and a corrector for correcting amplitude and phase mismatch caused by the I/Q modulator. The arrangement has means for sampling the I/Q modulated test signal to be transmitted, means for A/D converting the signal samples taken from the test signal, means for I/Q demodulating the signal samples digitally into I/Q feedback signals, and means for determining the correction parameters of phase and amplitude on the basis of the determined phase and amplitude mismatch.
Another related art of transmitter mismatch correction scheme utilizes an envelope detector and circuitry to detect the output of the transmitter and to amplify detected envelopes. For sinusoidal I/Q inputs at BB_I and BB_Q, the high-frequency envelope detector generates a filtered and amplified baseband ripple with spectral components at FBB due to LOFT and at 2×FBB due to I/Q mismatch. The phase and amplitude information can then be used to pre-distort the modulated signal.
These related arts only consider and calibrate the amplitude and phase mismatches, and these mismatches due to the mixers (e.g. 113 and 123) and the local oscillating signals (e.g. LOIt and LOQt) are generally frequency-independent. However, in reality, even though the amplitude and phase mismatches are well-calibrated, the communication circuit 100 still has RF imperfection. This is because frequency-dependent mismatches are not taken into consideration. Therefore, in wide-band applications, problems due to the I/Q mismatch of the communication circuit 100 will appear again. Such frequency-dependent mismatches exist because there are delay time mismatches caused by differences between component characteristics of the DACs 111 and 121, LPFs 112 and 122.